On Fri, Jun 15, 2018 at 12:50 PM Eric Botcazou <ebotca...@adacore.com> wrote:
>
> Hi,
>
> the motivating example is the following program in GNU C:
>
> void foo (int len)
> {
> int a = 1;
> int b[len];
>
> void bar (void)
> {
> b[0] = a;
> a++;
> }
>
> bar ();
> }
>
> int main (void)
> {
> foo (4);
> }
>
> If you look at the debug info at -O0 -g, you'll see that that the compiler
> generates 2 DIEs for variable "a" and 4 DIEs for variable "b" because:
> 1. It duplicates DIEs for each nested function containing up-level references
> 2. It generates 2 DIEs for each VLA subject to up-level references.
>
> That's sort of OK for languages where nested functions and VLAs are somewhat
> pathological devices but clearly doesn't scale for languages where they are
> both first-class citizens like Ada. That's why AdaCore has been using a
> totally different scheme in its toolchain, first using a dedicated encoding
> with STABS then using standard DWARF-2 only.
>
> This scheme is as follows:
> 1. The compiler generates only 1 DIE per variable as in the regular case,
> 2. For each child function, the static link is passed and the compiler
> generates a DW_AT_static_link attribute pointing to it for the child function,
> 3. The debugger reconstructs the static nesting tree at run time by walking
> up the static chain and thus can resolve up-level references on its own.
>
> It turns out that 2. and 3. were contributed a few releases ago, mainly by
> Pierre-Marie, both for GCC and GDB, so the last remaining step is 1. and it's
> the object of this contribution. Note that it also subsumes:
> https://gcc.gnu.org/ml/gcc-patches/2018-05/msg00873.html
> since contributing it piecewise turned out more problematic than helpful.
>
> So the patch entirely removes the support of artificial variables for debug
> info of up-level reference purposes present both in the Fortran FE and the
> gimplifier (the Ada FE has never had such a thing) but only disconnects it in
> the nested function lowering pass because it uses for another purpose by GOMP.
>
> Bootstrapped/regtested (GCC and GDB) on x86-64/Linux, OK for mainline?
Looks good to me. Rather than removing dwarf2/pr37726.c can you try
turning that into a guality test that verifies the debug experience is the
same (or better) than before? I realize guality stuff is fragile but you can
restrict it to -O0 if you like (not sure if dg-skip-if supports that).
Thanks,
Richard.
>
> 2018-06-15 Eric Botcazou <ebotca...@adacore.com>
>
> * gimplify.c (nonlocal_vlas): Delete.
> (nonlocal_vla_vars): Likewise.
> (gimplify_var_or_parm_decl): Do not add debug VAR_DECLs for non-local
> referenced VLAs.
> (gimplify_body): Do not create and destroy nonlocal_vlas.
> * tree-nested.c: Include diagnostic.h.
> (use_pointer_in_frame): Tweak.
> (lookup_field_for_decl): Add assertion and declare the transformation.
> (convert_nonlocal_reference_op) <PARM_DECL>: Rework and issue an
> internal error when the reference is in a wrong context. Do not
> create a debug decl by default.
> (note_nonlocal_block_vlas): Delete.
> (convert_nonlocal_reference_stmt) <GIMPLE_BIND>: Do not call it.
> (convert_local_reference_op) <PARM_DECL>: Skip the frame decl. Do not
> create a debug decl by default.
> (convert_gimple_call) <GIMPLE_CALL>: Issue an internal error when the
> call is in a wrong context.
> (fixup_vla_decls): New function.
> (finalize_nesting_tree_1): Adjust comment. Call fixup_vla_decls if no
> debug variables were created.
> * tree.c (decl_value_expr_lookup): Add checking assertion.
> (decl_value_expr_insert): Likewise.
>
>
> 2018-06-15 Eric Botcazou <ebotca...@adacore.com>
>
> * fortran/trans-decl.c (nonlocal_dummy_decl_pset): Delete.
> (nonlocal_dummy_decls): Likewise.
> (gfc_nonlocal_dummy_array_decl): Likewise.
> (gfc_get_symbol_decl): Do not call gfc_nonlocal_dummy_array_decl.
> (gfc_get_fake_result_decl): Do not generate a new DECL if simply
> reusing the result of a recursive call.
> (gfc_generate_function_code): Do not create, insert and destroy
> nonlocal_dummy_decls.
>
>
> 2018-06-15 Eric Botcazou <ebotca...@adacore.com>
>
> * gcc.dg/debug/dwarf2/pr37726.c: Delete.
> * gnat.dg/stack_usage5.adb: New test.
>
> --
> Eric Botcazou
